In the conventional art, this type of electromagnetic flow meter is configured such that an electric current whose polarity alternates with a prescribed frequency is supplied, as an excitation current, to an excitation coil, which is disposed such that the direction in which its magnetic field is generated is perpendicular to the direction in which the fluid flows inside a measurement tube. A frequency fex of the excitation current is called an excitation frequency.
Furthermore, supplying the excitation current at the excitation frequency fex to the excitation coil generates an emf (i.e., a signal emf) between a pair of electrodes that is disposed inside the measurement tube, the emf being orthogonal to the magnetic field generated by the excitation coil; furthermore, the measured flow can be obtained by detecting this signal emf as an analog flow signal and converting this detected analog flow signal to a digital signal.
In this electromagnetic flow meter, if foreign matter adheres to the electrodes, then a noise component owing to the adherence of this foreign matter will affect the signal emf, and it will no longer be possible to accurately measure the flow of the fluid (e.g., refer to Patent Document 1). Namely, the signal emf that arises between the electrodes will contain both the flow signal component and the noise component, the ratio of the noise component contained in the signal emf will increase, and it will no longer be possible to accurately measure the flow of the fluid.
Accordingly, if a function that automatically detects whether foreign matter is adhered to the electrodes (i.e., an electrode scaling detection function) is added to the electromagnetic flow meter, then removing the foreign matter can be performed in a timely manner, thereby improving the utility of the electromagnetic flow meter. Examples of electromagnetic flow meters that have such an electrode scaling detection function are disclosed in Patent Documents 2, 3.
In the electromagnetic flow meter described in Patent Document 2, the resistance of each electrode is measured, and if the resistance of a measured electrode exceeds a prescribed value (i.e., if an increase in the electrode resistance is detected), then it is judged that foreign matter is adhered to that electrode.
Two types of electromagnetic flow meters are described in Patent Document 3. In a first type of electromagnetic flow meter described in Patent Document 3, a ternary excitation system is adopted wherein excitation owing to excitation current in the positive direction is positive excitation, excitation wherein the excitation current is zero is nonexcitation, and excitation owing to excitation current in the negative direction is negative excitation; furthermore, based on the magnitude of signal emfs (V11−V15: signal emfs in the state wherein foreign matter is not adhered; V21−V25: signal emfs in the state wherein foreign matter is adhered) obtained at intervals K1−K5 (K1, K3, K5: nonexcitation; K2: positive excitation; and K4: negative excitation), calculation results R1−R4 (i.e., R1=−V21+V22+V23−V24, R2=(−V21+2V22−2V24+V25)/2, R3=−V11+V12+V13−V14, R4=(−V11+2V12−2V14+V15)/2) are calculated and, based on these calculation results R1−R4, a foreign matter adherence impact component is derived.
In the second type of electromagnetic flow meter described in Patent Document 3, a binary excitation system with two excitation frequencies (i.e., a working excitation frequency fH and a low excitation frequency fL) is adopted; furthermore, in the state wherein foreign matter is not adhered, a differential noise component is derived by subtracting the averaged process value of the signal emfs at the low excitation frequency fL from the averaged process value of the signal emfs during an interval at the working excitation frequency fH, and this derived differential noise component is stored in memory as a RAM variable A. Furthermore, in the state wherein foreign matter is adhered, a foreign matter adherence impact component is derived by subtracting the averaged process value of the signal emfs at the low excitation frequency fL from the averaged process value of the signal emfs during an interval at the working excitation frequency fH, and then subtracting from this value the RAM variable A (i.e., the differential noise component) stored in memory.